Nox hg and so2 removal using ammonia

ABSTRACT

A process and apparatus for removing SO?2#191, NO, and NO?2#191 from a gas stream having the steps of oxidizing ( 60 ) a portion of the NO in the flue gas stream to NO?2#191, scrubbing ( 62 ) the SO?2#191, NO, and NO?2#191 with an ammonia scrubbing solution, and removing ( 64 ) any ammonia aerosols generated by the scrubbing in a wet electrostatic precipitator. The process can also remove Hg by oxidizing it to HgO and removing it in the wet electrostatic precipitator. Ammonium sulfate, a valuable fertilizer, can be withdrawn from the scrubbing solution.

BACKGROUND OF INVENTION

a. Field of the Invention

This invention relates to methods and apparatuses for removing NOx andSO₂ from a gas stream.

b. Description of the Related Art

Fossil fuels are burned in many industrial processes. Electric powerproducers, for example, burn large quantities of coal, oil, and naturalgas. Sulfur dioxide (“SO₂”), nitrogen oxide (“NO”), and nitrogen dioxide(“NO₂”) are some of the unwanted byproducts of burning any type offossil fuel. Mercury (“Hg”) is often also found in fossil fuels. Thesebyproducts are known to have serious negative health effects on people,animals, and plants, and a great deal of research has been done to finda way to economically remove them from flue gas streams before theyenter the atmosphere.

SO₂ is often removed from gas streams (“desulfurization”) by scrubbingthe gas with an aqueous ammonium sulfate solution containing ammonia.Examples of this process are disclosed in U.S. Pat. Nos. 4,690,807,5,362,458, 6,277,343, and 6,221,325, which are not admitted to be priorart by their mention in this Background section. The absorbed sulfurcompounds react with ammonia to form ammonium sulfite and ammoniumbisulfite, which are then oxidized to form ammonium sulfate and ammoniumbisulfate. The ammonium bisulfate is further ammoniated to form ammoniumsulfate. The process does not remove NO or NO₂, however, which must thenbe dealt with using a different process.

NO and NO₂ (together known as “NOx”) can be removed from a gas stream bycontacting the gas stream with either ClO₂ or O₃ to convert NO into NO₂,and then scrubbing with an aqueous solution of a sulfur-containingreducing compound of alkali metals or ammonia, and a catalytic compound.Such a process is disclosed in U.S. Pat. No. 4,029,739, by Senjo et al.,which is not admitted to be prior art by its mention in this Backgroundsection. This process, however, does not remove SO₂, and requires theaddition of chlorine or ozone into the system by some other means.

Some processes exist that remove both NOx and SO₂. In one such processdisclosed in U.S. Pat. No. 4,035,470, by Senjo et al., which is notadmitted to being prior art by its mention in this Background section,NO is oxidized to NO₂ by contacting the gas with either ClO₂ or O₃ asabove. Then the SO₂ is scrubbed with a sulfite and an oxidationretardant that suppresses oxidation of the sulfite to sulfate. Iron orcopper compounds can also be added to depress oxidation. Optionally,ammonium hydroxide can be added to make sulfite and to react with CO₂ inthe gas stream to make carbonate. Like in U.S. Pat. No. 4,029,739mentioned above, this process requires the addition of either chlorineor ozone, and further requires a consumable sulfite oxidation retardant.The referenced patent did not mention whether the byproducts includedany valuable material like ammonium sulfate. However, both U.S. Pat.Nos. 4,029,739 and 4,035,470 require the addition of chlorine to a gasstream that is eventually released to the atmosphere, creating a serioussafety concern.

Yet another process for removing NOx and SO₂ from a gas stream isdisclosed in U.S. Pat. No. 4,971,777, by Firnhaber et al., which is notadmitted to be prior art by its inclusion in this Background section. Inthis process, NO is oxidized to NO₂ by the addition of organic compoundswhich decompose into radicals at high temperatures. Then an aqueousammonia solution in which the pH is adjusted to be below 5.0 absorbs theNOx and SO₂. Firnhaber teaches the importance of holding the scrubbingsolution to a low pH, since higher pH levels produce aerosols of theammonia salts that he says is an environmental burden to be thwarted.Ammonia aerosols are formed by gas phase reactions of ammonia vapor inthe scrubber and create a blue haze or white vapor that emanates fromthe stack. This is also called “ammonia slip.” Free ammonia in theatmosphere would be a serious health and environmental hazard. Firnhaberdismisses the possibility of aerosol removal means due to prohibitiveinvestment costs and high pressure loss, for instance.

What is needed, therefore, is a cost-effective process that removes SO₂,NO, and NO₂ from a gas stream that does not require the addition of acatalyst, chlorine, or ozone, can occur at relatively high pH, and doesnot result in ammonia slip.

SUMMARY OF INVENTION

The present invention is directed to a process and apparatus thatremoves SO₂, NO, and NO₂ from a gas stream that does not require theaddition of a catalyst, chlorine, or ozone, occurs at a relatively highpH, and does not result in ammonia slip. A process that satisfies theseneeds comprises the steps of oxidizing NO to NO₂, scrubbing SO₂, NO, andNO₂ from the flue gas stream with an ammonia scrubbing solution having apH between six and eight, and removing any ammonia aerosols generated bythe scrubbing steps with an aerosol removal means. These and otherfeatures, aspects, and advantages of the present invention will becomebetter understood with reference to the following description, drawings,and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a process flow chart showing the process of the presentinvention.

FIG. 2 is a cut-away view of an apparatus according to the presentinvention.

DETAILED DESCRIPTION

The present invention is a process and apparatus for removing SO₂, NO,and NO₂ from a gas stream, especially from the flue gas stream of afossil fuel boiler. In practice, flue gas from the combustion of fossilfuel nearly always contains more NO than NO₂, and often contains Hg,which can also be removed from the gas stream by this invention.

The inventors are familiar with methods and apparatuses for removing SO₂and NOx from gas streams. U.S. Pat. Nos. 5,871,703, and 6,117,403 teachthe use of an electrical discharge apparatus to oxidize SO₂ and NOx toform sulfuric and nitric acids respectively, collecting the acids in awet electrostatic precipitator (“WESP”) to form an effluent, andprocessing the effluent to make industrial grade acids that can be sold.The inventors on these two patents are Alix, Neister, and McLarnon, twoof whom are inventors of the present invention. U.S. Pat. No. 6,132,692teaches the use of a dielectric barrier discharge (“DBD”) reactor toform the same acids, collecting them in a WESP, and draining them fromthe WESP to remove them from a gas stream. The inventors on this patentare Alix, Neister, McLarnon, and Boyle, two of whom are inventors of thepresent invention. The above three patents were owned by the owner ofthe present invention as of the filing date of this specification. Theyare hereby incorporated by reference as if completely rewritten herein.

The present invention comprises a three-step process as shown in FIG. 1.A gas stream comprising SO₂, NO, NO₂, and perhaps Hg, are present priorto the first step 60. The first step 60 is oxidizing at least a portionof the NO in the flue gas to NO₂ with an oxidizing means. The meansselected should be able to oxidize greater than about two percent of theNO to NO₂, and is preferably in the region of about ninety percent.

The oxidizing step should be adjusted so that the resulting mole ratioof SO₂ to NO₂ after the oxidizing step should be at least 2.5 to 1. Theratio is preferably four to one, but can be greater. The oxidizing means60 can be any means known in the art, including but not limited to usingan electrical discharge reactor, and injecting ClO₂, O₃ or certainorganic compounds. For example, U.S. Pat. Nos. 4,029,739 and 4,035,470teach converting NO to NO₂ by the addition of ClO₂ or O₃ into the gasstream. U.S. Pat. No. 4,971,777 teaches the addition of certain organiccompounds that decompose into radicals at high temperatures.

Examples of suitable electrical discharge reactors include corona,pulsed corona, e-beam, and DBD. DBD is synonymously referred to assilent discharge and non-thermal plasma discharge. It is not the same ascorona discharge or pulsed corona discharge. The preferred embodimentuses a DBD reactor, such as that disclosed in U.S. Pat. No. 6,132,692,by Alix, et al. In practice, the operator of the process will adjust thepower input to the reactor to attain the desired oxidation results as afunction of the cost of power input to the reactor, desired scrubbingresults, and other factors. Laboratory testing has shown that oxidationof at least 90% of the NO and Hg is readily attainable with the presentinvention.

As taught in U.S. Pat. No. 6,132,692, a DBD reactor will oxidize atleast a portion of the NO and NO₂ in a gas stream to nitric acid, and atleast a portion of the SO₂ in a gas stream to sulfuric acid. These acidsare dealt with in the next step of the process.

If oxidizing means other than an electrical discharge reactor is used,Hg may or may not be oxidized to HgO. On the other hand, it is possible,and perhaps desirable, that some of the NO and NO₂ becomes furtheroxidized to form HNO₃ regardless of the means used. The reason why thismay be desirable will be made clear later in this specification.

Another oxidizing means 60 is adding ethylene or propylene to the fluegas followed by oxidizing NO to NO₂ in the electrical discharge reactor.This would have the advantage of reducing the power input requirement ofthe electrical discharge reactor to get the same amount of NO to NO₂oxidation. Ethylene can be added in about a 2:1 molar ratio of ethyleneto NO. The chemical reaction mechanisms for ethylene conversion of NO toNO₂ in an electrical discharge reactor are likely to be as follows:C₂H₄+OH-->HOCH₂CH₂  (1)HOCH₂CH₂+O₂-->HOC₂H₄OO  (2)NO+HOC₂H₄OO-->NO₂+HOC₂H₄O  (3)HOC₂H₄O+O₂-->HOCH₂CHO+HO₂  (4)NO+HO₂-->NO₂+OH  (5)In any event, the output gas stream comprises less NO, more NO₂, SO₂,perhaps HNO₃, perhaps H₂SO₄, and perhaps HgO, as shown in FIG. 1.

The second step 62 is scrubbing at least a portion of the SO₂, NO, andNO₂ present in the gas stream with an aqueous ammonia scrubbingsolution. The term “scrubbing” typically means “absorbing” to peoplehaving skill in the art, meaning that SO₂, NO, and NO₂ is absorbed bythe aqueous solution. However, it is intended that the term“scrubbing”as used in this specification also includes adding anhydrousammonia gas to initiate the reactions leading to the oxidation of SO₂and reduction of NO₂.

The solution preferably comprises ammonia, ammonium sulfite, ammoniumsulfate, and water. The solution preferably has a pH between six andeight, which is much higher than that taught by Firnhaber. Firnhaberteaches that the pH must be kept to less than five, and is preferably4.5, to prevent the formation of aerosols. However, the presentinvention is not concerned with avoiding the formation of aerosolsbecause it includes an aerosol removal means 64, described later in thisspecification.

Maintaining a relatively high pH has several benefits. It increases thespeed of absorption of SO₂. It increases the ratio of sulfite availablein solution compared to bisulfite, which facilitates the oxidation ofSO₂ and reduction of NO₂. The ratio of sulfite to bisulfite is highlydependent on pH level. From these benefits, it follows that theabsorption vessel, shown as item 44 in FIG. 2, can be substantiallysmaller than that used to scrub the same amount of SO₂ in a conventionallimestone scrubber which is the most typical SO₂ scrubber in use today.In addition, the amount of scrubbing liquid required and the liquid togas ratio can be reduced. It is estimated that the size of theabsorption vessel 44 can be reduced by half, and the liquid to gas ratiocan be reduced by a third. Because the cost of the absorption vessel andliquid circulating equipment represent a large fraction of the totalcost of a scrubber, the ability to substantially reduce the size of thevessel and associated pumps and piping is a major advantage of thepresent invention over the prior art.

Although FIG. 1 shows ammonia being added at this step, ammonia in theform of ammonium hydroxide can be added instead. The ammonia reacts withthe gas stream output from the oxidizing step, forming ammonium sulfiteand ammonium bisulfite. The likely chemical reactions in this step areas follows:NH₃+H₂O+SO₂-->NH₄HSO₃  (6)NH₄HSO₃+NH₃-->(NH₄)₂SO₃  (7)2NH₄OH+SO₂-->(NH₄)₂SO₃+H₂O  (8)

An oxidation inhibitor can be added at this step to inhibit theoxidation of sulfite to sulfate before the sulfite can perform its NO₂reduction function. Examples of oxidation inhibitors include thiosulfateand thiourea.

The ammonium bisulfite and ammonium sulfite reacts with the NO and NO₂to form ammonium sulfate. Ammonium sulfate is well known as a valuableagricultural fertilizer. The likely reactions that take place in thisstep are as follows:2NO₂+4(NH₄)₂SO₃-->4(NH₄)₂SO₄+N₂  (9)NO+NO₂+3(NH₄)₂SO₃-->3(NH₄)₂SO₄+N₂  (10)

Most of the HNO₃ that may have been formed by further oxidation of NOand NO₂, and/or created by a DBD reactor, will react with ammonia andform ammonium nitrate, also known to be a valuable agriculturalfertilizer, according to the following formula:HNO₃+NH₃-->NH₄NO₃  (11)

In a similar way, most of the sulfuric acid created by the DBD reactorwill react with the solution and form ammonium bisulfate and ammoniumsulfate. As one can see from the above equations, the process removesSO₂, NO, and NO₂ from the gas stream, and produces ammonium nitrate,ammonium sulfate, and nitrogen. Over time, the ammonium sulfate andammonium nitrate will concentrate in the aqueous ammonia solution andprecipitate out of solution. The solid precipitate can then be removedfrom the scrubber and processed for use as fertilizer.

The gas stream after the scrubbing step comprises nitrogen and water.Since the pH of the scrubbing solution is higher than about five, theoutput from the scrubbing step will likely contain ammonia aerosols. Ifnot collected in the scrubbing solution, the gas stream will alsocontain HgO.

The third step 64 is removing at least a portion of the ammonia aerosolsand the HgO, if present, from the gas stream. A wet electrostaticprecipitator (“WESP”) may be used as the aerosol removal means. A WESPis effective at collecting ammonia aerosols, HgO, and any other aerosolsor particles that may be present in the gas stream.

As a result of this three-step process, SO₂, NO, NO₂, and Hg are removedfrom a gas stream to provide ammonium sulfate and ammonium nitrate. Theoutput of the aerosol removal means comprises N₂ as a result of theprocess of the present invention.

An apparatus according to the present invention is shown in FIG. 2. Agas stream comprising SO₂, NO, NO₂, and perhaps Hg 14 enters theapparatus assisted by a forced draft fan 12. The gas then enters a meansfor oxidizing 10 at least a portion of the NO in the gas stream to NO₂.The oxidation means 10 performs the oxidizing step 60 shown in FIG. 1,which is more fully described above. In the preferred embodiment, atleast one DBD reactor is used, and can be provided in modules 16 tofacilitate manufacture and installation. At least one power supply andcontroller is required to operate a DBD reactor, which are selected bythose having skill in the art, but are not shown in the drawings.

After the oxidation means 10, the gas stream 18 comprises SO₂, less NO,more NO₂, perhaps HNO₃, perhaps H₂SO₄ and perhaps HgO. The gas streamtemperature at this point is about 177° C. (350° F.). The gas streamthen enters a scrubbing vessel 44 in a region 19 over an aqueousammonium sulfate solution 22. Preferably, the aqueous ammonium sulfatesolution comprises ammonia, ammonium sulfite, ammonium sulfate, andwater. Water in the ammonium sulfate solution 22 evaporates due to theheat of the gas stream 18, thus concentrating ammonium sulfate solution15, which is then removed from the vessel 44. The removed ammoniumsulfate solution 15 can processed by industry standard means to producea saleable fertilizer product.

Air or other oxidizers 17 may be introduced into the ammonium sulfatesolution 22 for oxidizing ammonium sulfite into ammonium sulfate.Ammonium sulfate solution 22 is pumped with a circulation pump 50 to aset of lower spray nozzles 24 that serve to cool and saturate the gasstream 18 with water vapor, and to a bubble cap tray 36 to absorbammonia vapors.

Another circulation loop is provided wherein aqueous ammonium sulfiteand sulfate in a vessel 48 is pumped with a circulation pump 52 to a setof upper spray nozzles 34. The liquid then falls to a dual flow tray 30.A separator tray 26 allows some of the liquid to fall into the ammoniumsulfate solution 22, and the remainder is piped to the vessel 48.Additional makeup ammonia 32 is added to the upper spray nozzles 34.These two circulation loops, independently or together, perform thescrubbing step 62 of FIG. 1, which is described in detail above.

Following the scrubbing loops, a WESP 40 is provided to remove anyammonia aerosols or HgO that may have formed earlier in the process. TheWESP 40 is preferably a shell-and-tube type of WESP, but can be a platetype, or any WESP such as is known by those having skill in the art. TheWESP 40 is wetted using a set of sprays 42 fed with water via a conduit20. A mist eliminator 38 can be provided below the WESP 40. The WESP 40is an example of the aerosol removal means 64 described in FIG. 1. Thegas stream 46 exiting the WESP 40 has considerably less NOx and SO₂ thanthat which entered the process and apparatus, and has an increasedamount of the reaction products, which are nitrogen and water.

The following laboratory-scale examples of the process demonstrate theefficacy of the present invention:

EXAMPLE 1

An absorption test was done for the scrubbing step of the process of thepresent invention, with a solution that was 1% w/w SO₃ ²⁻ (“sulfite”),6% w/w SO₄ ²⁻ (“sulfate”), and 2.5% S₂O₃ ²⁻ (“thiosulfate”) in a packedcolumn that was 46 cm (18 inches) high and 3.8 cm (1.5 inches) indiameter. The column was packed with 0.64 cm ({fraction (1/4)} inch)glass RASCHIG rings. The simulated flue gas at the inlet of the columncontained 13% v/v moisture, 6% v/v O₂ and the simulated flue gaspollutants listed in the table. There was continuous addition of NH₃ and(NH₄)₂S₂O₃ to maintain a pH of 6.8 and a thiosulfate concentration of2.5% w/w. The residence time in the column was 1.8 sec with an L/G ratioof 56 lpm/kacm·hr (25 gpm/kacfm).

The table shows the concentrations of NO, NO₂, and SO₂ at the inlet andoutlet of the test system. TABLE 1 Scrubbing Step Alone System InletSystem Outlet NO (ppmv) 20 4 NO₂ (ppmv) 250 36 SO₂ (ppmv) 1370 2

EXAMPLE 2

An absorption test was done for the scrubbing step of the process of thepresent invention starting with water and a flue gas stream consistingof 13% v/v moisture, 17 ppmv NO, 267 ppmv NO₂, 1360 ppmv SO₂, 6% v/v ° 2and balance N₂. Ammonia and ammonium thiosulfate were added to maintaina pH of 6.8 and a thiosulfate concentration of 2.5%, and theconcentrations of sulfite and sulfate in the system were allowed tobuild to steady state. The NOx removal rate was 80% w/w atconcentrations of SO₃ ²⁻, SO₄ ²⁻ and S₂O₃ ²⁻ of 0.7% w/w, 2.5% w/w, and0.5% w/w respectively.

EXAMPLE 3

Tests were conducted in a laboratory test facility for the NO oxidizing,scrubbing, and aerosol removal steps of the process of the presentinvention. The equipment consisted of a simulated flue gas deliverysystem, a coaxial cylinder DBD reactor, a packed column scrubber and atubular WESP. The following is an example of data obtained in the labtest facility. Simulated flue gas was delivered to the DBD reactor at aflow rate of 14 scfm, a temperature of 290° F. and with the followingcomposition: 6.2% v/v O₂, 14.2% v/v CO₂, 8.2% v/v H₂O, 20 ppmv CO, 250ppmv C₂H₄, 1740 ppmv SO₂, and 259 ppmv NO_(x). Gas velocity through thedischarge reactor was 15 m/s (50 ft/sec) with discharge power level of140 watts. Gas from the discharge reactor entered a 10 cm (4 inch) IDpacked column scrubber, packed with 1.3 cm ({fraction (1/2)} inch)INTALOX saddles to a depth of 1.2 m (4 feet). Liquid was introduced atthe top of the scrubber at a flow rate of 1.2 lpm (0.33 gpm), L/G=44lpm/kacm·hr (20 gpm/kacfm). Aqueous ammonia was added to and effluentliquid removed from the recirculating scrubber solution to maintain aconstant total liquid volume and solution pH at 6.6. Gas from the packedbed scrubber was treated in a 10 cm (4 inch) ID wetted wallelectrostatic precipitator with a gas residence time of 0.7 seconds. Thetable below shows the concentrations of NO, NO₂ and SO₂ at the inlet tothe system, the outlet of the barrier discharge reactor and at theoutlet of the system. TABLE 2 Three Step Process Discharge ReactorSystem System Inlet Outlet Outlet NO (ppmv) 254 45 32 NO₂ (ppmv) 5 109 9SO₂ (ppmv) 1740 1598 1

The three-step process and apparatus described herein was designedspecifically to treat flue gas from a coal fired power plant. However,it can be appreciated that the invention is capable of operating on anygas stream in which NOx and SO₂ are present, including but not limitedto gas and oil-fired boilers and various chemical manufacturingprocesses. The NOx and SO₂ concentrations and operating conditions willbe different in each situation. Therefore, it is understood that anoperator or system designer will be motivated to modify the scrubbingstep 62 to possibly eliminate the need for either one or both theoxidizing step 60 or the aerosol removal step 64, or combine the threeelements somehow so that fewer than three steps are needed.

It will be apparent to those skilled in the art that various changes andmodifications can be made without departing from the spirit of thepresent invention. Accordingly, it is intended to encompass within theappended claims all such changes and modifications that fall within thescope of the present invention.

1. A process for removing SO₂, NO, and NO₂ from a gas stream comprisingthe steps of a. oxidizing at least a portion of NO in a gas stream toNO₂ with an oxidizing means resulting in a mole ratio of SO₂ to NO₂ ofbetween 2.5 to 1 and 4 to 1, followed by b. scrubbing at least a portionof SO₂, NO, and NO₂ from the gas stream with a scrubbing solutioncomprising ammonia, and having a pH between 6 and 8, and c. removing atleast a portion of any ammonia aerosols generated from the scrubbingstep from the gas stream with an aerosol removal means.
 2. The processof claim 1, wherein said oxidizing means is an electrical dischargereactor.
 3. The process of claim 2, wherein said electrical dischargereactor is a dielectric barrier discharge reactor.
 4. The process ofclaim 3, further comprising the step of oxidizing at least a portion ofthe NO to HNO₃ with said dielectric barrier discharge reactor.
 5. Theprocess of claim 1, wherein said oxidizing means comprises injectingethylene or propylene.
 6. The process of claim 1, wherein said oxidizingstep is adapted to result in a mole ratio of SO₂ to NO₂ of at least fourto one.
 7. The process of claim 1, said scrubbing solution comprisingammonia, ammonium sulfite, ammonium sulfate, and water, and having a pHbetween 6 and
 8. 8. The process of claim 1, wherein said aerosol removalmeans is a wet electrostatic precipitator.
 9. The process of claim 1,wherein said scrubbing step results in the formation of ammoniumsulfate, the process further comprising the step of withdrawing ammoniumsulfate from the scrubbing solution.
 10. The process of claim 4, whereinsaid scrubbing step results in the formation of ammonium nitrate, theprocess further comprising the step of withdrawing ammonium nitrate fromthe scrubbing solution.
 11. A process for removing SO₂, NO, NO₂, and Hgfrom a gas stream comprising the steps of a. oxidizing at least aportion of the NO in a gas stream to NO₂, and at least a portion of theHg in a gas stream to HgO, with an oxidizing means resulting in a moleratio of SO₂ to NO₂ of between 2.5 to 1 and 4 to 1, followed by b.scrubbing at least a portion of the SO₂, NO, and NO₂ from the gas streamwith a scrubbing solution comprising ammonia, and having a pH between 6and 8, and c. removing at least a portion of any ammonia aerosolsgenerated from the scrubbing step, and HgO, from the gas stream with anaerosol removal means.
 12. The process of claim 11, wherein saidoxidizing means comprising a dielectric discharge reactor.
 13. Theprocess of claim 11, wherein said oxidizing means comprising injectingethylene or propylene.
 14. The process of claim 11, wherein said aerosolremoval means is a wet electrostatic precipitator.
 15. The process ofclaim 11, said scrubbing solution comprising ammonia, ammonium sulfite,ammonium sulfate, and water, and having a pH between 6 and
 8. 16. Theprocess of claim 15, wherein said scrubbing step results in theformation of ammonium sulfate, the process further comprising the stepof withdrawing ammonium sulfate from the scrubbing solution.
 17. Anapparatus for removing SO₂, NO, and NO₂ from a gas stream comprising a.an oxidizing means for oxidizing at least a portion of the NO in a gasstream to NO₂, followed by b. a scrubber suitably adapted to scrub atleast a portion of the SO₂, NO, and NO₂ from the gas stream with ascrubbing solution comprising ammonia, and having a pH between 6 and 8,and c. an aerosol removal means for removing at least a portion of anyammonia aerosols generated by the scrubber from the gas stream.
 18. Theapparatus of claim 17, wherein said oxidizing means is at least oneelectrical discharge reactor.
 19. The apparatus of claim 18, whereinsaid electrical discharge reactor is at least one dielectric barrierdischarge reactor.
 20. The apparatus of claim 19, wherein saiddielectric barrier discharge reactor is adapted to oxidize at least aportion of the NO to NO₂ and HNO₃.
 21. The apparatus of claim 17, saidscrubbing solution comprising ammonia, ammonium sulfite, ammoniumsulfate, and water, and having a pH between 6 and
 8. 22. The apparatusof claim 17, wherein said aerosol removal means is at least one wetelectrostatic precipitator.
 23. An apparatus for removing SO₂, NO, NO₂,and Hg from a gas stream comprising a. an oxidizing means for oxidizingat least a portion of the NO in a gas stream to NO₂, and at least aportion of the Hg in a gas stream to HgO, followed by b. a scrubbersuitably adapted to scrub at least a portion of the SO₂, NO, and NO₂from the gas stream with a scrubbing solution comprising ammonia, andhaving a pH between 6 and 8, and c. an aerosol removal means forremoving at least a portion of any ammonia aerosols generated by thescrubber, and HgO, from the gas stream.
 24. An apparatus for removingSO₂, NO, and NO₂ from a gas stream comprising a. an NO oxidizer adaptedto oxidize at least a portion of the NO in a gas stream to NO₂, followedby b. a scrubber adapted to scrub at least a portion of the SO₂, NO, andNO₂ from the gas stream with a scrubbing solution comprising ammonia,and having a pH between 6 and 8, and c. an aerosol remover adapted toremove at least a portion of any ammonia aerosols generated by thescrubber from the gas stream.
 25. The apparatus of claim 24, whereinsaid NO oxidizer is at least one electrical discharge reactor.
 26. Theapparatus of claim 25, wherein said electrical discharge reactor is atleast one dielectric barrier discharge reactor.
 27. The apparatus ofclaim 26, wherein said dielectric barrier discharge reactor is adaptedto oxidize at least a portion of the NO to NO₂ and HNO₃.
 28. Theapparatus of claim 24, said scrubbing solution comprising ammonia,ammonium sulfite, ammonium sulfate, and water, and having a pH between 6and
 8. 29. The apparatus of claim 24, wherein said aerosol remover is atleast one wet electrostatic precipitator.